IIRC, Yamata no Orochi is lvl 34... however, Pyro Jack (used before) is lvl 32, meaning MC started around lvl 30 already
At the fusion... considering MC has Pyro Jack on the Magician side, I guess he used at least
Ganga (lvl 29) on the Priestess side... that is... supposing the anime tries to follow the basic fusion rules...
... you know when you played too much Persona 4 when you manage to calculate the result of a normal spread fusion after years of playing said game